Polytetrafluoroethylene (PTFE) grafts placed into the arterial circulation of baboons for 8 weeks under high blood flow (HF) conditions develop a thin intima composed of smooth muscle cells (SMCs) and extracellular matrix beneath an endothelial monolayer. When these grafts are returned abruptly to normal flow (NF), they develop marked intimal thickening within 1 month. The mechanisms underlying this thickening are unclear. We studied the SMC response to altered flow by placing bilateral aortoiliac PTFE grafts into baboons with bilateral femoral arteriovenous fistulas. After 8 weeks, one fistula was closed, returning the graft flow on that side to NF. The opposite graft remained under HF conditions. Flow differences were monitored with duplex ultrasound (for all grafts: NF, 135±21 [mean±SEM] mL/min; HF, 507±35 mL/min; P<.001). Grafts were removed 2, 4, 7, 14, or 28 days later (five animals per group). Endothelial coverage, as assessed by scanning electron microscopy, was intact in each graft. Intimal area and SMC number increased progressively in NF grafts through 28 days (for area: NF, 3.0±0.3 mm2; P<.001; and for SMCs per cross section: NF, 11.8±1.1x103; HF, 2.6±1.Ox103; P<.002). Intimal SMC proliferation (thymidine labeling) was increased significantly in NF grafts at 4 and 7 days (at 4 days: NF, 5.9±1.5%; HF, 1.4±0.6%; P<.05). Extracellular matrix accounted for an equal proportion of intimal mass in NF and HF grafts (percent matrix at 28 days: NF, 62.9±1.6%; HF, 63.7±4.7%; P=NS). We conclude that intimal thickening in this model of flow-induced vascular remodeling is due to increased SMC proliferation and accumulation of SMCs with a proportionate amount of extracellular matrix.